The Diagnostic Value of ECG Characteristics for Vasospastic and Microvascular Angina: A Systematic Review

ABSTRACT Background Coronary vascular dysfunction comprises VSA and/or MVA and is more common in women than in men with angina without obstructive coronary artery disease (ANOCA). Invasive coronary function testing is considered the reference test for diagnosis, but its burden on patients is large. We aimed to investigate the potential of electrocardiography (ECG) as noninvasive marker for vasospastic angina (VSA) and microvascular angina (MVA) diagnosis. Methods We systematically screened Pubmed and EMBASE databases for studies reporting on ECG characteristics in ANOCA patients with (a suspicion of) coronary vascular dysfunction. We assessed study quality using QUADAS‐2. We extracted data on diagnostic values of different ECG characteristics and analyzed whether the studies were sex‐stratified. Results Thirty publications met our criteria, 13 reported on VSA and 17 on MVA. The majority addressed repolarization‐related ECG parameters. Only 1 of the 13 VSA papers and 4 of the 17 MVA papers showed diagnostic accuracy measures of the ECG characteristics. The presence of early repolarization, T‐wave alternans, and inverted U waves showed of predictive value for VSA diagnosis. The QTc interval was predictive for MVA diagnosis in all six studies reporting on QTc interval. Sex‐stratified results were reported in only 5 of the 30 studies and 3 of those observed sex‐based differences. Conclusions ECG features are not widely evaluated in diagnostic studies for VSA and MVA. Those features predictive for VSA and MVA diagnosis mostly point to repolarization abnormalities and may contribute to noninvasive risk stratification.

Ischemic heart disease affects 126 million people in the world (Khan et al. 2020).Obstructive coronary artery disease (CAD) is absent in 40%-70% of patients with symptoms and signs of myocardial ischemia undergoing coronary angiography (Jespersen et al. 2012).Recent studies show that the prevalence of coronary vascular dysfunction (microvascular angina [MVA] and/or vasospastic angina [VSA]) ranges between 84% and 89% among patients with persistent angina and nonobstructive CAD (ANOCA) (Ford et al. 2019;Suda et al. 2019;Konst et al. 2021).The endotype prevalence differs between sexes, with a higher prevalence of microvascular spasm and coronary microvascular dysfunction and a lower prevalence of epicardial spasm in women compared to men (Jansen et al. 2021).
The current reference standard to diagnose MVA and VSA is an invasive coronary function test (CFT).A CFT involves the administration of intracoronary acetylcholine (Ach) or ergonovine (EG) for the detection of coronary spasm.In addition, coronary flow reserve (CFR) and microvascular resistance assessment in response to adenosine using a dedicated guidewire technique is used to assess the microvascular function (Ford et al. 2018).Even though the initial presenting symptoms of these patients may be similar, the treatment and outcome depend on the endotype diagnosed (Ford et al. 2018(Ford et al. , 2020)).These findings have inspired cardiologists to perform more invasive tests which are now also recommended in clinical guidelines (Knuuti et al. 2020;Gulati et al. 2021).As a result, a broader ANOCA population is eligible to undergo invasive testing for medical treatment to be targeted to the specific underlying pathophysiological endotype.However, CFTs are of high burden for the patient and healthcare system.Unfortunately, the best patient selection strategy for CFT remains unclear, and practice standards differ across and even within countries.This highlights the need for a low-risk non-invasive diagnostic test for risk stratification to optimize the selection of patients in whom further testing is required.
The electrocardiogram (ECG) is an essential diagnostic tool for various cardiac pathologies, in particular myocardial ischemia.Also, ECGs are important for ischemia detection during the CFT for the diagnosis of VSA.In a nonacute setting, ECG changes were observed in cardiac diseases linked to coronary vascular dysfunction (e.g., QTc prolongation in patients with heart failure with preserved ejection fraction [Cho et al. 2021]).The ECG may therefore be a valuable noninvasive diagnostic tool for risk stratification in ANOCA patients.
We hypothesize that VSA/MVA patients compared to patients who have a noncardiac origin of their complaints present with specific ECG changes, and that these ECG changes have added value in the diagnosis of coronary vascular dysfunction in ANOCA patients.We therefore performed a systematic review presenting the current evidence for the use of ECG for the diagnosis of VSA and/or MVA in women and men with ANOCA.

| Data Sources and Search Strategy
The search was conducted on July 7, 2021 and updated on July 29, 2022.We combined the search results from the PubMed and EMBASE databases.The search terms included words related to "electrocardiography," "coronary microvascular dysfunction," "coronary vasospasm," "angina pectoris and nonobstructive coronary artery disease," "vasomotor dysfunction" and variants on these terms (limited to title and abstract), MeSH headings, and Emtree terms.The full search strategy is listed in Method S1.We registered the protocol of this systematic review in PROSPERO (https:// www.crd.york.ac.uk/ prosp ero/ ; registration number: CRD42022336911).

| Study Selection
The search results were filtered to only find Dutch and English written records.After the removal of duplicates, two independent researchers (DS and JW) screened the records by title and abstract and subsequently on their full text.Only studies performed on human adults and publications for which the full text was available were considered for inclusion.All types of ECG measurements as index tests were considered eligible.Case reports, conference papers/abstracts, and editorials were excluded, as were studies that did not include patients with ANOCA, or that did not study ECG characteristics.DS and JW resolved disagreements by discussion.Figure 1 shows the search and selection processes.

| Data Extraction
DS and JW extracted the study characteristics of interest of all included papers using a standardized data collection form.The study characteristics were the year of publication, name of the first author, country, study design, total number of participants, percentage of women in the study, average age, inclusion and exclusion criteria, description of the study group and reference group, type of ischemic heart disease, prevalence of outcome, diagnostic method/definition of the outcome, ECG measurement method, ECG features studied, and results.The definition of VSA or MVA used per study was checked against the Coronary Vasomotion Disorders International Study Group (COVADIS) criteria for a definitive diagnosis or suspicion of VSA and MVA (Beltrame et al. 2017;Ong et al. 2018).In addition, it was noted whether or not the results were stratified by sex.Reporting was done taking the PRISMA-DTA reporting guidelines into account (McInnes et al. 2018).The data are presented separately for the two coronary vascular dysfunction endotypes (VSA and MVA).Furthermore, the level of evidence of the ECG features was rated.The level of evidence was low, moderate, or high if similar results for the ECG feature were shown in less than two cross-sectional studies, two cross-sectional studies, or more than two cross-sectional studies, respectively.

| Critical Appraisal
DS and JW independently performed the methodological quality assessment using the QUADAS-2 tool (Whiting et al. 2011).The risk of bias was assessed on four key domains: patient selection (Domain 1), index test (Domain 2), reference standard (Domain 3), and flow and timing (Domain 4).We added the following question to Domain 2 to improve the risk of bias assessment of the index test: "Was the index test administered in the same way in all groups if a case-control design was used instead of a cross-sectional design?"(Table S1).Publications were scored for their risk of bias (low/high/ unclear) and concerns of applicability (low/medium/high/ unclear).Disagreements were again resolved by discussion between DS and JW.

| Search Results
The search resulted in 6785 records, and the selection resulted in 30 papers that met the predefined inclusion and exclusion criteria as depicted in Figure 1.Of the 30 papers included in the final analysis, 13 (43%) focussed on VSA, 17 (57%) focussed on MVA, and none focussed on the combined endotype (VSA and MVA).

| Quality Assessment
The risk of bias for the VSA papers was generally low for Domains 2 (index test), 3 (reference standard), and 4 (flow and timing) (Table 1).The risk of bias and applicability concern in Domain 1 (patient selection) were frequently high in these papers.The most common reason was the use of a case-control study design and the use of (healthy) symptom-free controls.Furthermore, we allocated a medium or high applicability concern for Domain 2 when the aim of the study was not (exactly) the same as was aimed for in this review (i.e., the diagnostic value of an ECG characteristic for VSA or MVA).The interval between the index test (ECG) and reference standard was often not reported in both VSA and MVA papers, resulting in an unclear risk of bias (Domain 4).In addition, a high risk of bias for Domain 4 (second column) was allocated to some papers, since not all study subjects received the reference standard.Many MVA papers had a high risk of bias or applicability concern for Domain 3 (reference standard), since only a few studies used invasive coronary reactivity testing to diagnose MVA.We did not exclude studies based on their risk of bias or applicability concerns.

| Vasospastic Angina
Table 2 shows an overview of the characteristics of the 13 studies describing ECGs of patients with and without VSA.Six   S1).publications used a cross-sectional study design (n = 24-827 subjects, VSA prevalence between 33% and 57%), and seven used a case-control study design (n cases = 7-50; n controls = 8-50).

Year of publication
Most studies included fewer women than men (mean: 33% women; range: 0%-58%).Only one study reported sex-stratified results (Matsumoto et al. 1989).One article reported on the diagnostic performance of the ECG characteristic using measures of accuracy (i.e., AUC, positive predictive value [PPV], and negative predictive value [NPV], and/or sensitivity and specificity) (Matsumoto et al. 1989).The majority of the studies (n = 11, 85%) used an invasive spasm provocation test with Ach or EG as the reference test to diagnose VSA.In only 6 of the 13 studies, the diagnostic criteria were in accordance with the COVADIS criteria for definitive VSA diagnosis (Beltrame et al. 2017).In the 13 papers, 20 ECG features were analyzed (Table 3), including 11 heart rate variability-related (n = 3 papers), two QRS-related (n = 2 papers), two J-wave-related (n = 3 papers), one QT-related (n = 1 paper), one T-wave-related (n = 2 papers), and three Uwave-related (n = 3 papers) parameters.

| Heart Rate Variability-Related Parameters
Heart rate variability (HRV) is considered a measure that reflects the balance between sympathetic and parasympathetic activity of the autonomic nervous system (Yamasaki et al. 1996).
A low HRV can therefore indicate reduced parasympathetic activity or less commonly increased sympathetic activity.Three case-control studies (cases, n = 7 to 35; controls, n = 9 to 19; Yamasaki et al. 1996;Tsuchiya et al. 1996;Yoshio et al. 1993) focused on HRV analysis.The controls consisted of healthy subjects or subjects with atypical chest pain without any heart disease.Time-domain parameters are based on a signal (in this case HRV) over time, whereas the frequency domain parameters describe how much of the signal (the power) lies within certain frequency bands.VSA patients and controls had no differences in high-frequency domain HRV parameters or the ratio between low-frequency and high-frequency power (Table 3).The results for mean RR interval and low-frequency domain parameters were inconsistent.In one study, the global standard deviation of all normal RR intervals over 24-h (SDNN) and the percentage of differences between adjacent normal RR intervals >50 ms computed during the entire 24-h ECG (pNN50) were slightly but significantly lower in VSA patients compared to controls (SDNN: 124.2 ± 37.6 ms vs. 149.2± 29.7 ms (p < 0.05), respectively; pNN50: 5.6 ± 4.6% vs. 9.4 ± 4.1% (p < 0.05), respectively) (Tsuchiya et al. 1996).Therefore, the evidence for a predictive value of time domain and low-frequency domain HRV parameters for VSA is very limited (Table 3) (Yamasaki et al. 1996;Tsuchiya et al. 1996;Yoshio et al. 1993).

| QRS-Related Parameters
Two cross-sectional studies showed results on QRS-related parameters, that is, late potentials and QRS axis shift (Tables 2 and  3) (Matsumoto et al. 1989;Akiya et al. 1997).The term "late potentials" is used to describe the presence of electrical activity at the end of the QRS complex.The frequency of late potentials was 38% in patients with a positive provocation test (n = 50) and 23% in patients with no stenosis and a negative provocation test (n = 82) (ns, The green, orange, red, and gray colors represent low, medium, high, and unclear risk of bias or applicability concern, respectively.Abbreviations: MVA, microvascular angina; VSA, vasospastic angina.p = 0.07) (Akiya et al. 1997).Right QRS axis shift showed a sensitivity and specificity of 70% and 100% for men, 0% and 50% for women, and 58% and 80% for men and women combined (n = 24, 33% women), respectively (Table 3) (Matsumoto et al. 1989).

| J-Wave-Related Parameters
Both the Brugada syndrome and the early repolarization syndrome are referred to as J-wave syndromes and are associated with an increased risk of cardiac arrhythmias.The investigators of a cross-sectional study (n subjects = 827, VSA prevalence: 39%) tested the co-existence of Brugada syndrome and VSA (Table 2) and did not demonstrate such co-existence (Table 3) (Ong et al. 2011).In two other cross-sectional studies (n subjects = 116-286, VSA prevalence: 33%-57%), investigators explored the early repolarization syndrome in VSA and non-VSA subjects, defined as J-point elevation ≥1 mm above baseline and notching or slurring of the terminal QRS portion in at least two inferior and/or lateral leads.The frequency of early repolarization was higher in the VSA group (29%-36%) than in the non-VSA group (6%-12%) (Table 3) (Ikeda et al. 2020;Inamura et al. 2015).Furthermore, the odds of VSA increased in case of the following early repolarization characteristics: early repolarization in inferior leads, notched type, and horizontal/descending ST segments and higher amplitudes (>0.2 mV) (Ikeda et al. 2020).The level of evidence for early repolarization is moderate (Table 3).

| QT-Related Parameters
One case-control study (50 patients and 50 controls with atypical chest pain and a negative stress and spasm provocation test) focused on QTc dispersion (Table 3) (Suzuki et al. 1998).They defined QTc dispersion as the difference between the minimum and maximum QT intervals (corrected for heart rate) of a 12lead ECG.VSA patients had statistically significantly greater baseline QTc dispersion compared to controls (69 ± 24 ms vs. 44 ± 19 ms (p < 0.001), respectively) (Suzuki et al. 1998).

| T-Wave-Related Parameters
T-wave alternans (TWA) is beat-to-beat variation in the shape and amplitude of the T wave.One cross-sectional (Inamura et al. 2015) and one case-control (Shimada et al. 2012) study focused on TWA (Table 3).Both studies measured TWA of ambulatory ECG recordings by using the modified moving average method (Nearing and Verrier 2002).The incidence of positive TWA (value >65 μV) was higher in the VSA group (44%-60%) compared to subjects without VSA (11%) (Inamura et al. 2015) or controls that were age-and gender-matched to the VSA patients (0%) (Shimada et al. 2012).

Level of evidence
Frequency of late potentials (%) (Akiya et al. 1997) Higher    waves was higher in the VSA group (79%-89%) compared to the non-VSA group (4%) (Yano et al. 1987) or the control group (0%) consisting of patients with atypical chest pain without CAD or coronary spasms (Igarashi et al. 1995) (Table 3).According to one study, all U-wave inversions in the VSA group were terminal U-wave inversions (U-wave inversion after positive U-wave deflection), whereas all U-waves were initial U-wave inversions (U-wave inversion proceeded to positive U-wave deflection) in a control group of patients with hypertension without CAD (Table 3) (Miwa et al. 1993).However, the level of evidence for a predictive value of the presence of (terminal) negative and inverted U waves for VSA is low.No sex-differentiated information was provided.

| Microvascular Angina
Table 4 shows an overview of the characteristics of the 17 studies focussing on MVA.Of the 17 publications, five described a cross-sectional study design (n = 59-926 subjects, MVA prevalence between 19% and 43%) and 12 case-control studies (n cases = 14-261; n controls = 14-261).The median percentage of women included in the studies was 64% (range 31%-100%).
The percentage of women was unknown for one article.Three studies included only women.Four papers (24%) stratified their results for sex.Only five papers (29%) used invasive coronary reactivity testing with adenosine (n = 4 papers) or dipyridamole (n = 1 paper) to diagnose microvascular angina.Two (12%) used transthoracic Doppler echocardiography with dipyridamole and one (6%) a positron emission tomography (PET) scan with 13 N-ammonia or 82 rubidium.In a majority of the papers (n = 9, 53%), the diagnosis MVA was based on a combination of symptoms and various tests (at least an exercise test and coronary angiography) to assess the presence of myocardial ischemia or rule out obstructive CAD or coronary vasospasm.In none of the studies, the diagnostic criteria were in accordance with the COVADIS criteria for definitive MVA diagnosis (Ong et al. 2018).Thirty-eight ECG features were analyzed in the 17 publications (Table 5), including 16 heart rate variabilityrelated (n = 7 papers), three P wave-related (n = 3 papers), two QRS-related (n = 3 papers), two QRS-T-related (n = 1 paper), four QT-related (n = 6 papers), one ST-segment-related (n = 3 papers), and ten T-wave-related (n = 3 papers) parameters (Table 5).

| Heart Rate Variability-Related Parameters
Two cross-sectional studies and five case-control studies compared heart rate (variability) features between MVA patients and controls or patients with a normal CFR (>2.0 or 2.5) (Tables 4 and 5) (Sara, Lennon et al. 2016;Spinelli et al. 1990;Galassi et al. 1991;Rosano et al. 1994;Lee et al. 1996;Ozcan et al. 2021;Mammana et al. 1997).All controls consisted of healthy subjects.One study also had a CAD control group (Galassi et al. 1991).Since the CAD group falls outside our domain, further reference will only be made to the healthy control group for this study.MVA patients had higher (diurnal) heart rates (∆ = 4-11 bpm) compared to controls or patients with normal CFR (>2.0 or 2.5) (Table 5) (Sara, Lennon et al. 2016;Galassi et al. 1991;Ozcan et al. 2021;Mammana et al. 1997).
However, in one of the studies, the heart rate was only higher among female MVA patients and not among male MVA patients (Sara, Lennon et al. 2016).One of the case-control studies showed lower RR interval deviations over 24 h in MVA patients compared to controls (Rosano et al. 1994).Differences in heart rate were not seen on the ECG during exercise (Table 5) (Spinelli et al. 1990).Furthermore, MVA patients had lower diastole percentage of the cardiac cycle (±3.8%)at different levels of exercise compared to controls and shortening of the diastolic time at each exercise level compared to the predicted values based on the observed heart rate (Spinelli et al. 1990).In another study, MVA patients had a less steep heart rate/time slope during exercise than controls (3.3 ± 0.8 vs. 4.2 ± 1.1 bpm, respectively, p < 0.003) (Galassi et al. 1991).Regarding frequency domain HRV parameters, MVA patients had lower values of total power, low-frequency domain and high-frequency domain parameters and higher low-frequency/high-frequency power ratio compared to controls (Table 5) (Rosano et al. 1994;Lee et al. 1996).
Hence, evidence for a predictive value of heart rate variability parameters for MVA is scarce.

| P-Wave-Related Parameters
In three cross-sectional studies, investigators compared the PR interval (n = 3 papers), P-wave duration (n = 1 paper), and prevalence of atrial fibrillation (n = 1 paper) between patients with normal and abnormal CFR (Sara, Lennon et al. 2016;Ozcan et al. 2021;Dose et al. 2018).One of these studies was designed as a case-control study but also included a cross-sectional part.We will only describe the results from the cross-sectional part here (Table 4) (Dose et al. 2018).None of the studies reported on sensitivity and specificity outcome measures.PR interval and P-wave duration did not differ between patients with normal and abnormal CFR (Table 5) (Sara, Lennon et al. 2016;Ozcan et al. 2021;Dose et al. 2018).The prevalence of atrial fibrillation was 32% in patients with abnormal CFR (<2.0) and 15% in patients with normal CFR (ns, p = 0.07) (Table 5) (Ozcan et al. 2021).

| QRS-Related Parameters
A case-control study showed higher frequency of fragmented QRS complexes (fQRS) in the ECGs of MVA patients (30%) compared to controls (4%, p = 0.001) (  5).Some evidence exists against QRS duration and very little evidence exists for fQRS to have predictive value for MVA.

| QRS-T-Related Parameters
Ventricular gradient (VG) and total R T cosine (TRTC) are parameters that reflect the direction of depolarization and repolarization.The VG is the three-dimensional magnitude and angle of the QRS complex and T-wave area in the vector ECG.TRTC quantifies the difference between depolarization and repolarization wavefront directions, expressed as mean cosine of       QT (Rosen et al. 1994) No difference between MVA patients and controls (38 ± 19 vs. 34 ± 9 ms, p = ns) QTc dispersion defined as difference between maximal and minimal QTc interval (Lee et al. 1998) No differences between MVA patients and controls (37 ± 11 vs. 36 ± 10 ms, p = ns).A QTc dispersion ≥50 ms in response to standing + a positive exercise test: sensitivity = 62%, specificity = 77%.QTc       the angles between QRS and T vectors in a three-dimensional space.VG and TRTC reacted attenuated to postural changes and the Valsalva maneuver (tests to examine autonomic function) in MVA patients (n = 16) compared to healthy subjects (n = 40) (Table 5) (Batchvarov et al. 2002).

| QT-Related Parameters
QTc interval was the most studied ECG parameter of all included MVA studies (6/17 studies of which three cross-sectional).In all six studies, MVA patients had QTc interval prolongation (mean/ median ranging between 4 and 30 ms) in comparison to controls or subjects with a CFR > 2.0 or CFR > 2.5 (Table 5) (Sara, Lennon et al. 2016;Ozcan et al. 2021;Mammana et al. 1997;Dose et al. 2018;Rosen et al. 1994;Lee et al. 1998).The results were sex-stratified in only two of the studies.In one casecontrol study, female MVA patients and controls had higher QTc intervals than male MVA patients and controls (Rosen et al. 1994).In a cross-sectional study, prolongation of the QTc interval was only present among males and not among females (Sara, Lennon et al. 2016).The level of evidence for QTc interval prolongation is high.
QT(c) dispersion results are equivocal (Table 5).The QT dispersion did not differ between MVA patients and controls in one study (Rosen et al. 1994) whereas in another study, QTc dispersion ≥50 ms in response to standing plus the presence of a positive treadmill exercise test or in response to postural changes showed relatively high sensitivity (62% and 73%, respectively) and specificity (77% and 67%, respectively) for MVA diagnosis (Table 5) (Lee et al. 1998).
The results indicate that the absence of ST segment depression in ECG can support the exclusion of MVA.

| T-Wave-Related Parameters
One cross-sectional study (Dose et al. 2018) and two case-control studies (Kaplan and Aksan 2016;Sara, Sugrue et al. 2016) focused on T-wave-related parameters.Regarding T-wave time aspects, Tpeak-Tend intervals differed between MVA patients and controls (Kaplan and Aksan 2016;Sara, Sugrue et al. 2016).In one study, the (corrected) Tpeak-Tend intervals and Tpeak-Tend/ QT(c) ratios were higher (Kaplan and Aksan 2016), whereas in another study, the results differed per ECG lead (Table 5) (Sara, Sugrue et al. 2016).They reported longer Tpeak-Tend intervals in aVL and shorter intervals in V5 and V6 in females.In males, they showed shorter Tpeak-Tend intervals in lead II and aVF (Table 5) (Sara, Sugrue et al. 2016).T-wave asymmetry, T-wave flatness, and the presence of T-wave notch do not seem to have predictive value for MVA, since a combined numeric score of these ECG features showed no differences between MVA (CFR < 2.0) and patients with higher CFR (

| Discussion
This systematic review presents the current evidence on ECG characteristics and their predictive value for VSA or MVA in patients with ANOCA.In this review, we observed that the focus of most papers was on ventricular repolarization-related components of the ECG (J-waves, ST segments, and T-and U-waves) and fewer on depolarization-related ECG parameters, for both VSA and MVA.We document that ECG characteristics are not widely evaluated in diagnostic studies of both VSA (Matsumoto et al. 1989) and MVA (Lee et al. 1998;Youn et al. 2005;Lopez et al. 2021;Sara, Sugrue et al. 2016).Furthermore, only 5 (17%) of the included studies reported sex-stratified data, and in 3 of the 5 studies sex-based differences were discovered.Consistent findings for VSA were higher incidence of positive T-wave alternans, higher frequency of early repolarization and inverted U waves, and the absence of differences in high-frequency domain parameters in VSA patients compared to non-VSA patients or controls.Consistent findings for MVA patients compared to non-MVA or controls were higher QTc interval, lower values for total power, LF and HF domain HRV-related parameters, and the absence of differences in PR interval and QRS duration.The results indicate that it may be possible to discriminate VSA from MVA using the ECG, since other ECG characteristics showed of predictive value for VSA (i.e., T-wave alternans, early repolarization, and inverted U waves) than for MVA (i.e., QTc interval).The low number of studies investigating each ECG characteristic, the low sample sizes in the majority of the included studies, and the low reporting of accuracy measures make the evidence on the diagnostic performance of the ECG characteristics weak.(Matsumoto et al. 1989;Akiya et al. 1997).Negative U waves are mentioned as (early) clinical markers of ischemia but also occur in combination with hypertension or valvular regurgitation (Yano et al. 1987;Igarashi et al. 1995).Although the mechanism behind inverted or negative U-wave genesis is still unknown, a possible mechanism is delayed repolarization of the His-Purkinje system (Correale et al. 2004).HRV parameters are accepted as indices of autonomic nervous activity.The high-frequency component and low-frequency component are thought to be indices of parasympathetic activity and sympathetic activity, respectively (Yamasaki et al. 1996).Therefore, the results of our review suggest increased sympathetic nervous activity in VSA patients (Boudou et al. 2017).The involvement of the sympathetic nervous system in the genesis of vasospasms is plausible, because this system plays an important role in the vascular tone through multiple mechanisms including direct vasoconstriction (Bruno et al. 2012).

| ECG Characteristics for VSA
Considering most results indicate changes in repolarizationrelated ECG parameters, we recommend to focus on repolarization-related ECG parameters for future diagnostic studies on VSA.

| ECG Characteristics MVA
Around one third of the publications reported higher values for the QTc interval in MVA patients compared to non-MVA patients or controls.Although these results indicate that the QTc interval has predictive value for MVA, none of these papers reported diagnostic accuracy measures.Also, higher heart rates (possibly only in females), higher presence of ST depression, and differences in Tpeak-Tend interval between MVA patients compared to non-MVA patients and controls were observed, but only in two to three studies per ECG characteristic.PR interval and QRS duration differences were clearly absent between MVA patients and non-MVA patients.
ECG characteristics related to myocardial ischemia or infarction, such as fQRS and ST depression, are promising to investigate as MVA patients have myocardial ischemia (Damar et al. 2014;Youn et al. 2005;Roy et al. 2020;Shehata et al. 2019).Myocardial ischemia impacts ventricular repolarization (Klabunde 2017), which can be interpreted from research by Verrier and Ikeda (2013), Verrier, Nearing, and D' Avila (2021).This can explain differences in QTc time, T-wave duration and morphology parameters, ventricular gradient, and total R T cosine in patients with MVA.However, differences in ECG characteristics can have many other causes.For example, QTc time can also be influenced by medication, genetic or electrolyte disorders, diabetes, or a prolonged QRS duration (Rautaharju, Surawicz, and Gettes 2009).The higher heart rates, shorter diastolic times, and lower values for low-frequency domain HRV parameters observed in patients with MVA in comparison to controls may indicate a difference in autonomic nervous activity.Since coronary flow in the subendocardial regions occurs mainly in the diastolic phase, shortening of the diastolic period can be one of the causes of regional ischemia in patients with MVA (Spinelli et al. 1990).Also, clinical syndromes associated with coronary microvascular dysfunction can contribute to ECG differences between patients with and without MVA.An example of such a clinical syndrome is heart failure with preserved ejection fraction, of which left ventricular diastolic dysfunction is considered a precursor (Camici et al. 2020).ECG features like increased QTc interval, ST segment deviation, or increased Tpeak-Tend intervals can also be observed in patients with left ventricular diastolic dysfunction (Van Ommen et al. 2021).
Indeed, most results point to changes in repolarization-related ECG parameters in MVA patients.Therefore, studies may focus on repolarization-related ECG parameters in diagnostic studies for MVA which seem to be the most promising based on this review.

| Quality
The wide range of ECG features and low reporting on diagnostic performance limits the comparability of the results.Many of the included studies used a case-control study design, with controls often consisting of healthy subjects or patients without any chest pain complaints.Only two case-control studies used controls sampled from the same study base as the cases (Table 1; Kaplan and Aksan 2016;Sara, Sugrue et al. 2016).Furthermore, information on the control group was scarce in some publications.
Case-control studies with wrong control groups lead to difficulties interpreting the detected associations.Using healthy control subjects will probably result in an overestimation of the associations.To be able to diagnose VSA/MVA patients in the clinical practice, VSA/MVA patients must not be compared to healthy people, but to patients who are referred for the same indication (ANOCA) but who have a noncardiac origin of their complaints.
The variation in reference standard for VSA diagnosis was low, since most studies used invasive spasm provocation testing to diagnose VSA, although using heterogeneous provocation protocols.However, the reference standard used for MVA diagnosis showed large heterogeneity, where in 12 of the 17 studies, patients were not evaluated using the diagnostic reference standard (Table 4).In 13 of the 30 studies, a proportion of patients did not even receive the reference standard used within the study (Table 1).In addition, in most studies, the diagnostic criteria for VSA or MVA were not in accordance with the COVADIS criteria, which is largely related to the fact that the COVADIS criteria had not been reported at the time of the publications (Beltrame et al. 2017;Ong et al. 2018).In seven VSA studies, the diagnostic criteria were not in accordance with the COVADIS criteria.This was due to the unreported extent of coronary artery constriction or the use of more lenient vasoconstriction thresholds of less than 90% diameter reduction to diagnose clinically significant spasms.None of the MVA studies were in accordance with the COVADIS criteria, since most studies did not specifically provide evidence of impaired coronary microvascular function and for the studies that did provide evidence of impaired coronary microvascular function, it was unclear whether the criteria of objective evidence of myocardial ischemia was met.Furthermore, the only marker of impaired coronary microvascular function in these studies was an impaired coronary flow reserve.It is therefore not known whether coronary microvascular spasm also plays a role in these patients.As a result of these considerations, it should be noted that the patients in the included papers mainly fall in the group of "suspected" of VSA or MVA according to the currently available COVADIS criteria.
The type of ECG measurement used within the included studies was also heterogeneous (Table 2).In most of the VSA papers, investigators studied ECGs measured during spasm provocation testing.Although it seems plausible that similar ECG characteristics emerge during spontaneous episodes or even that precursors are present on routinely obtained ECGs, the performance of such ECG features on ECGs needs further investigation.Our results contribute to identifying possible ECG characteristics for the diagnosis of VSA and MVA in the future.The ECG measurement methods used within the MVA studies can be roughly divided into 12-lead rest ECG, 12-lead ECG during exercise, and 24-h Holter measurements.In terms of type of ECG measurement, the findings on ECG features for MVA diagnosis are therefore more generalizable for use as noninvasive diagnostic.However, the prognostic value of these ECG types will probably differ, with a higher prognostic value for exercise ECGs compared to rest ECGs (Stocco et al. 2018).Besides, in many papers, the time interval between the index test and reference standard is unclear (Table 1).This results in an unclear risk of bias.ECGs of periods before disease onset or after disease reduction due to treatment could potentially be used, but this may cause misclassification.

| Limitations
The results are based on limited data since many included studies had a small sample size, and the studies had little overlap in studied ECG characteristics.Furthermore, only a few papers (14%) reported on the diagnostic performance of the studied ECG characteristic.Therefore, a formal metaanalysis could not be performed.In addition, publication bias could not be assessed.Another limitation is the low number of publications (n = 5, 17%) reporting sex-specific results.For most ECG features, it is therefore unclear whether they could be helpful in VSA/MVA prediction in both men and women.Furthermore, no papers reported on ECG characteristics in the combined endotype (VSA and MVA).A third of ANOCA patients undergoing CFT is affected by the combined endotype (Jansen et al. 2021).It is therefore important that this patient group is also included in research concerning diagnosis in ANOCA patients.For VSA, most publications contained data from studies in Japan, because VSA research is focused on that region.It has been described that Japanese patients have hyperreactive coronary arteries and a higher incidence of multivessel spasms in comparison to Caucasian patients (Beltrame, Sasayama, and Maseri 1999).The results may therefore not be generalizable.

| Future Research
The included publications consist of many small case-control studies, while a cross-sectional study design is essentially the most valid study design for diagnostic accuracy studies.Furthermore, only a small percentage of the publications (14%) reported on the diagnostic performance of the ECG characteristic by a measure of accuracy.Measures of accuracy provide us with information on the discriminative or predictive ability of a diagnostic test for a certain disease.We therefore recommend that future studies on the diagnostic performance of ECG characteristics use a cross-sectional study design with sufficient sample size and report on diagnostic performance by measures of accuracy.In addition, we encourage the use of reporting checklists, like the STARD (STAndards for the Reporting of Diagnostic accuracy studies) Checklist (Cohen et al. 2016).The use of such a checklist can substantially improve the reporting quality of diagnostic studies and therefore improve the evidence provided.At last, we recommend the report on sex-specific results.Of the VSA papers, only one article reported on sex-specific results and showed that in contrast to men, the potential ECG characteristic (right QRS axis shift) did not occur in any of the women with a positive spasm provocation test.This ECG characteristic would therefore probably not be applicable for VSA diagnosis in women.This example demonstrates the importance for investigating and reporting findings in a sex-specific manner.

| Conclusion
This systematic review presents the current evidence for the use of ECG for the diagnosis of VSA or MVA in patients with ANOCA.The identification of noninvasive diagnostic ECG characteristics of VSA and MVA is important in view of the increasing population with these abnormalities.The most promising ECG features for VSA and MVA prediction are mainly related to repolarization abnormalities.To disclose latent ECG features, it can be helpful to do provocative testing during the ECG measurement, particularly in patients with MVA.However, ECG features are not widely evaluated in diagnostic studies for VSA and MVA.Due to the low number of studies investigating each ECG

FIGURE 1 |-
FIGURE 1 | Flow diagram of the systematic search and study selection processes.

TABLE 1 |
Critical appraisal in accordance with the adjusted QUADAS-2 criteria (Table

TABLE 2 |
Overview of the characteristics of the vasospastic angina studies.

TABLE 3 |
ECG features studied in VSA patients.

TABLE 4 |
Overview of the characteristics of the microvascular angina studies.
Abbreviations: CAD, coronary artery disease; CAG, coronary angiography; CFR, coronary flow reserve; CMD, coronary microvascular dysfunction; CSX, cardiac syndrome X; ECG, electrocardiogram; EG, ergonovine; MVA, microvascular angina; PET, positron emission tomography; RG, reference group; TRTC, total R T cosine; VG, ventricular gradient.aOnly the data of the cross-sectional part of the publication are used for this systematic review.

Table 5
Sugrueet al. 2016).Thus, T-wave area, Y-center of gravity (of the first 25% of the T wave), right T slope, T-wave amplitude, and Tpeak-Tend interval may have added value in the diagnosis of MVA.
(Inamura et al. 2015;Suzuki et al. 1998tive value of T-wave alternans, early repolarization, and inverted U waves for VSA.However, none of the studies investigating these ECG parameters reported diagnostic accuracy measures and not all of the ECG parameters were evaluated in well-designed diagnostic studies.Differences in some time domain or low-frequency domain HRV parameters, late potentials, right QRS axis shift, and baseline QTc dispersion are present in patients with VSA compared to non-VSA patients or controls.The level of evidence for the predictive value of these ECG parameters was low, as these parameters were only studied in one or no cross-sectional studies.To increase insight into the diagnostic value of these parameters, larger diagnostic test accuracy studies are necessary.High-frequency domain HRV parameters and Brugada ECG criteria did not show promise for VSA diagnosis.Early repolarization is thought to be caused by enhanced local early repolarization.Recently, however, it was shown that early repolarization can also be caused by local conduction delay(Boukens, Potse, and Coronel 2021).Early repolarization can therefore be caused by multiple factors, such as myocardial ischemia or structural abnormalities(Ikeda et al. 2020;Inamura et al. 2015).The higher TWA and greater baseline QTc dispersion in VSA patients compared to controls or the non-VSA group are likely due to inhomogeneity of ventricular repolarization(Inamura et al. 2015;Suzuki et al. 1998).The higher incidence of late potentials and right QRS axis shift can be explained by slow conduction and hemiblock, respectively, caused by myocardial ischemia